An optical head applies a laser beam to an optical disk for optically recording information on a storage medium such as an optical disk, and reads recorded information through the reflected state of the laser beam. The optical head must have two functions. The first is an emitting function for applying a laser beam to an accurate position on a disk in an accurately focused state and, second, a signal detecting function for detecting the light reflected from the disk in the form of an electric signal. Information is stored on an optical disk in accordance with known methods. To read information, it is necessary to apply light accurately to the track and detect reflected light continuously. In the following description, a disk is referenced as a typical optical storage medium.
The emitting function is defined as a function for emitting a laser beam for recording and reproducing information to a disk serving as a storage medium, accurately adjusting the focus of the laser beam by moving an objective lens along the optical axis by an actuator to focus the laser beam on a fine area on the optical storage medium, and, tracking the laser beam to a track where a desired signal is recorded.
This operation must be performed continuously while the laser beam is emitted to the optical storage medium from the optical head because fluctuation occurs in a range of approximately 100 .mu.m in the optical axis direction during rotation of the optical storage medium, in view of the planar accuracy of the medium, and a fluctuation range of about 50 to 100 .mu.m in the radial direction of the medium surface during rotation of the medium, in view of the track forming accuracy. Therefore, it is necessary to continuously move the objective lens, by using the actuator, to apply the laser beam to the disk by keeping correct focus and a correct tracking state.
Information for detecting correct focus and a correct tracking state is included in the light reflected from the disk in the form of its intensity, wave front state, and polarization. The reflected light also includes information recorded on the optical recording medium. These pieces of information are read by a signal detecting optical system which includes information detecting optics, focus signal detecting optics, and track signal detecting optics. Focus and tracking information is fed back to the actuator and used as information for servo operation to have the laser beam correctly follow the track. Hereafter, a signal detected from light reflected from the disk as information for the servo operation is referred to as a servo signal.
Unless each of these detecting systems is set to a proper position when the optical head is manufactured, the servo signal cannot be accurately detected. Therefore, the servo operation cannot be performed for accurately applying the laser beam to the track of the optical recording medium.
In the case of the conventional optical disk drive, the diameter of the focused spot on a medium is approximately 0.9 .mu.m, and the pitch between data tracks on it is 1.6 .mu.m. As a result, an accuracy of approximately .+-.1 .mu.m is required in the direction of the optical axis in the case of focus and an accuracy of approximately .+-.0.1 .mu.m in the radial direction in the case of tracking. High-accuracy positioning of the focused spot is realized by the servo technique and high accuracy is required for servo signal detection for focusing and tracking. Therefore, as described above, highly accurate mechanical positioning is also required for assembling the signal-detecting optical systems of an optical head. In general, it is necessary to adjust the position of each part to an accuracy of several microns during assembly.
In assembling an optical head, the position of each detector in the optical head 920 has been determined by using an adjustment reference disk 901 as shown in FIG. 9 to optimize the detected servo-signal condition.
FIG. 2 is a schematic view of a separate optics-type optical head for the magneto-optical disk to be adjusted. A laser beam emitted from a laser diode 201 is converted to parallel rays by a collimator lens 203 and, moreover, converted to a shape close approaching a perfect circle by a beam circularizer 205; the direction of the beam is also changed. Then, the laser beam passes through a beam splitter 207, its direction is changed again by a reflective mirror 204 and the beam is thereafter focused by an objective lens 210 and applied to a reference disk 250. The laser beam reflected by the reference disk 250 include data signal and servo signals for focus and tracking, and returns to the beam splitter 207 through the same path. Then, the beam splitter 207 splits the reflected light in the direction of each detector for detecting focus and tracking signals and data information.
In FIG. 2, the focus state is detected by a quad-split detector 233 in branched reflected light. Reflected light passes through an amorphic lens 231 before it is introduced to the quad-split detector 233. Similarly, the tracking state is detected by a far-field detector 211 and the information stored in tracks is detected by an MO signal detector 225 after passing through a 1/2-wave plate 221 and a polarized beam splitter 223.
The focus state is described below in detail. The focus state is detected when reflected light reaches the quad-split detector 233 through the amorphic lens 231. The amorphic lens is defined as a lens whose focus differs in the x and y axes drawn on a plane vertical to the optical axis. To make the quad-split detector serving as a mechanism for detecting the focus state properly show its function, the amorphic lens and quad-split detector must be arranged at a predetermined interval in the direction of the optical axis. In this case, a positional error in the amorphic lens of no more than several tens of microns is permitted with regard to the quad-split detector. Also, a positional error of no more than several tens of microns or less in the x and y directions is permitted for the quad-split detector in the plane vertical to the optical axis.
The far-field detector 211 serves as a mechanism for detecting the tracking state. To make the detector 211 correctly detect the tracking state, however, a positional error in the detector 211 of no more than several hundreds of microns is permitted in each direction in the plane vertical to the optical axis.
For a magneto-optical (MO) disk, information is recorded on a recording medium by heating track spots on the disk surface with the laser beam with the disk being in a magnetic field. The direction of magnetization is detected in accordance with the polarized state of the disk surface based on the Kerr effect upon application of the laser beam. This is performed by detecting a light signal split by the polarized beam splitter 223 through the 1/2-wave plate 221 by the MO signal detector 225. Also, in the case of detection by the MO signal detector 225, the polarized direction of signal detection light must be rotated up to a predetermined angle in accordance with the state of the 1/2-wave plate 221, and the MO signal detector must be arranged at a predetermined position. In this case, it is necessary to limit the error of the rotation angle of the 1/2-wave plate to several degrees and the positional error of the MO signal detector in x and y directions on the plane vertical to the optical axis to several hundred microns or less.
Therefore, the step of assembling an optical head requires a process (tracking) for accurately setting the position of each detector. Conventionally, a reference disk is used to position detectors as shown in FIG. 9. In this case, a laser beam is applied to the disk to determine the position of each detector so that the detection state is optimized by monitoring the reflected light of the laser beam by a proper means.
At the stage of positioning, to obtain positioning information using a reference disk for adjustment, a servo signal must show the focus or tracking state. In realizing the above mentioned, a light spot must be properly focused on a track of the disk. At the beginning of this adjustment, however, the position of each detector is very roughly set and the position of each detector not yet optimized. The positions of detectors are then adjusted repeatedly until signals are optimized. Therefore, it is very difficult to obtain accurate information by detecting the servo signal for focus and tracking with the light reflected from the reference disk.
To be specific, a method is used in which focus and/or tracking signals are observed, respectively, and adjustments are adjusted one by one so that the position of each component approaches a point where all signals are optimized. However, both the focus and tracking signals are generated simultaneously by a reference disk for adjustment, and it is difficult to separately generate one from the other, and mutual interference of the signals cannot be avoided. Moreover, an adjustment frequently influences a plurality of signals because of the structure of the optical head. Therefore, it is estimated that a problem lies in the method for adjustment using a reference disk.